The absence of an invasive air sac system in the earliest dinosaurs suggests multiple origins of vertebral pneumaticity

The origin of the air sac system present in birds has been an enigma for decades. Skeletal pneumaticity related to an air sac system is present in both derived non-avian dinosaurs and pterosaurs. But the question remained open whether this was a shared trait present in the common avemetatarsalian ancestor. We analyzed three taxa from the Late Triassic of South Brazil, which are some of the oldest representatives of this clade (233.23 ± 0.73 Ma), including two sauropodomorphs and one herrerasaurid. All three taxa present shallow lateral fossae in the centra of their presacral vertebrae. Foramina are present in many of the fossae but at diminutive sizes consistent with neurovascular rather than pneumatic origin. Micro-tomography reveals a chaotic architecture of dense apneumatic bone tissue in all three taxa. The early sauropodomorphs showed more complex vascularity, which possibly served as the framework for the future camerate and camellate pneumatic structures of more derived saurischians. Finally, the evidence of the absence of postcranial skeletal pneumaticity in the oldest dinosaurs contradicts the homology hypothesis for an invasive diverticula system and suggests that this trait evolved independently at least 3 times in pterosaurs, theropods, and sauropodomorphs.

Specimens. The studied specimen of Buriolestes schultzi (CAPPA/UFSM 0035) corresponds to a nearly complete articulated skeleton and one of the best-known early dinosaurs 19 . It is one of the oldest sauropodomorphs and the basalmost taxon within this clade (but see 22 ). The holotype of Gnathovorax cabreirai (CAPPA/ UFSM-0009) corresponds to a nearly complete articulated skeleton, with some dorsal axial segments still under preparation 21 . It is one of the oldest and best-preserved herrerasaurids ever found to date. The holotype of Pampadromaeus barberenai (ULBRA-PV016) is an unarticulated almost complete skeleton 20 . Since the ULBRA collection was closed, ULBRA-PV016 is now housed in CAPPA/UFSM. Whereas histological thin sections were not performed, some structures demonstrate that all specimens reached an advanced degree of skeletal maturity, indicating that they were not juveniles at the time of death. Such evidence is based on the neurocentral sutures, the presence and shape of some muscle attachment structures, such as the femoral trochanteric shelf and anterolateral scar in the proximal portion of the femur, and dorsolateral trochanter [23][24][25] .  20 . Therefore, it is known that Pampadromaeus pertained to a faunal assemblage slightly more recent than that of Buriolestes and Gnathovorax (see Fig. 1).

Locality and horizon.
Taphonomic remarks. CAPPA/UFSM 0035 and 0009 were not significantly affected by taphonomic compression, whereas several bone elements of ULBRA-PV016 are diagenetically distorted, not reflecting their natural shape. Nevertheless, the external texture of the elements of ULBRA-PV016, as well as that of the other two specimens, is well-preserved, depicting fine details of its anatomy. Opaque diagenetic minerals are also present in CAPPA/UFSM 0035 and 0009 as well as moderate calcite permineralization.
Anatomical nomenclature. Wilson's nomenclature was applied for vertebral laminae and fossae 6,27,28 and Wedel's terminology for vertebral pneumatic structures 5,29,30 . We followed O'Connor's method to identify unambiguous evidence of PSP in our specimens 3 . Figure 1 exemplifies vertebrae of extinct and living taxa, and indicates structures such as laminae and fossae, illustrating different types of pneumatic and apneumatic architectures.
Micro-computed tomography (μCT scan). The anterior (third) cervical vertebra and an articulated posterior cervical vertebra of Gnathovorax were scanned. Dorsals are still under preparation in the jacket and were not available for this analysis. Almost the entire cervical series of Buriolestes was scanned as well as two middle dorsals. Finally, the three available dorsal vertebrae of Pampadromaeus were scanned. Specimens were scanned on a Bruker-Skyscan 1173 microtomographer equipped with a 130-kV μ-focus X-ray source with a voxel size of 0.15 mm. The procedure took place at the Instituto do Petróleo e dos Recursos Naturais at the Universidade Católica do Rio Grande do Sul/PUCRS), Porto Alegre (Brazil). 3D-Slicer v5.2 31 and Cloud-Compare v2.9.1 32 were used to analyze the data by applying color grades based on bone tissue density 33 . Digital measurements were taken with ImageJ v1.52 34 . All microtomography data was uploaded to the Morphobank platform and is available through this link: http:// morph obank. org/ perma link/? P4477.

Results
The neural arches of the three taxa show laminae and fossae with moderate complexity compared to what is observed in derived saurischians. Gnathovorax presents more robust vertebrae with deeper lateral fossae at the centra than the sauropodomorphs.  6H) and also laterally in the pedicles in Buriolestes (Fig. 4D). Two layered trabeculae (ltr) are present in the cotyles of both sauropodomorphs but not in the herrerasaurid. Pampadromaeus features a 'pseudo-polycamerate' (ppc) architecture (Fig. 6L). Here we define 'pseudopolycamerate' as chaotic apneumatic trabecular chambers infilled with blood and fat tissues but resemble the fractals of the pneumatic polycamerate as defined by Wedel 29 . Figure 7 illustrates both architectures side-by-side  www.nature.com/scientificreports/ in high-contrast monochromatic vertebral profiles. Something similar exists in the earlier Buriolestes but the configuration, in this case, is rather a combination of chaotic trabeculae and elongated chambers (Fig. 3L,M). Finally, the neural canal nutritional foramina are broader towards the dorsal-cervical joint, as shown in Buriolestes

Discussion
The microtomography of the three specimens evidences the absence of unambiguous PSP. Our data also support that a better description of the foramen anatomy can solve questions on ambiguous PSP 41   www.nature.com/scientificreports/ Proportionally large-diameter foramina (e.g., > 5 mm, in our specimens) associated with deep fossae must be taken into consideration when diagnosing unambiguous PSP 3,41 . Those latter must also be connected with clear internal pneumatic architecture 3,30,41 . Most pneumatic vertebrae are internally organized into camerate and camellate bone tissues, and these are the best macroscopic correlates of bone-penetrating pneumatic diverticula 11,13,29,36,37,42,43 . These Carnian taxa failed to show an architecture consistent with pneumatization, such as that observed in the derived sauropods Saltasaurus (PVL 4017-17/214) 11 and Ibirania (LPP-PV-0200) 37 , and theropods like Aoniraptor (MPCA-Pv 804/1-25) 43 and noasaurids (DGM 929-R) 36 . Even giant basal neosauropods   10 and Giraffatitan (MB.R. 2180.25/26) 44 present large organized corticaltissue camerae suggesting interaction with pneumatic diverticula. Lastly, our data indicate that an invasive pneumatic system was not present in the postcranial skeletons of the earliest Carnian dinosaurs. The earliest evidence of PSP occurred in the fossae adjacent to the diapophysis in most basal sauropodomorphs 45 . Our scans show that in Buriolestes, Pampadromaeus, and Gnathovorax any fossae associated with the neural arches or diapophyses are shallow, simple (i.e., not divided into subfossae sensu Wilson 27 ), and do not represent invasive pneumatization of the vertebrae. The evidence of the absence of PSP in the earliest dinosaurs suggests that invasive PSP found in theropods, sauropods and pterosaurs was not homologous 3,15,30,46,47 . This is solid evidence that invasive intraosseous pneumatization must have evolved at least three times independently (see Fig. 8). Our results are also in accordance with recent findings that demonstrate the nonexistence of PSP in the early ornithischian Heterodontosaurus 47 . Nonetheless, this still does not exclude the hypothesis indicating that the homology of the underlying noninvasive pulmonary tissue could be an ancestral ornithodiran condition 15,48 . In extant birds, many diverticula do not interact with the skeleton, including intermuscular, intervisceral, and subcutaneous diverticula 49 . However, similar 'cryptic' diverticula 50 in basal ornithodirans would be unlikely to preserve in the Triassic fossil record, and by definition, such diverticula do not leave diagnostic skeletal traces, and both of these factors complicate any attempts to investigate the earliest stages in the evolution of PSP. Even histological evidence of the respiratory diverticula requires interaction with the bone to survive fossilization 4,33,37,43,51 .
Some early non-archosaurian archosauriforms such as Erythrosuchus present developed fossae, laminae, and even foramina which led some authors to propose ancestral evidence of PSP 52 . However, we suggest a careful analysis of foramina size and position, as well as a detailed description of the internal trabecular architecture. These are the definitive macroscopic indicators of unambiguous PSP, as previously hypothesized 3,30,41 . Erythrosuchus microCT scan (NHMUK R3592) 15 revealed a chaotic array of trabecular walls in a very dense, near-granular, mass of thick bone throughout the vertebra (Fig. 6 in 15 ). Additionally, the absence of connections between internal structures with external foramina corroborates with the apneumatic status of Erythrosuchus 15 .
The short time and phylogenetic distances between the early sauropodomorph Buriolestes and the later Pampadromaeus allowed the observation of a short step in the evolution of vertebral vasculature. The increasingly irrigated internal architecture with chaotic trabecular vacancies and chambers in basal dinosaurs, filled with blood and fat tissues, could have favored the evolution of the true pneumatic structures in the latest Triassic. Pampadromaeus was excavated in a stratum slightly younger than Buriolestes and presented a pseudo-polycamerate vascularized architecture, much different from the chaotically organized pattern of the older Buriolestes. This change demonstrates the increasing complexity of these tissues that may have laid the groundwork for the later origin of true unambiguous PSP.
We note many suggestive correspondences between the apneumatic vertebral internal structure of these Carnian dinosaurs and the pneumatic internal structures of more derived saurischians. Similar apneumatic chambers were also described in the vertebral centra of non-avemetarsalian archosauromorphs such as in the centra of the Alligator (see Fig. 1C,D) and in the neural arches of taxa such as Erythrosuchus 15 . In early saurischians, in addition to the pseudo-polycamerate architecture discussed above, apneumatic chambers in the centra (ccv) of the Carnian taxa are structurally similar to pneumatic camerae of later sauropods and theropods (Fig. 6) 29 . Even more compellingly, small circumferential chambers (cc) and layered trabeculae (ltr) in these early dinosaurs resemble www.nature.com/scientificreports/ circumferential pneumatic chambers and radial camellae in derived titanosaurian sauropods 37 . In extant birds, developing pneumatic diverticula have been observed to follow pre-existing blood vessels 3,53 . If pneumatization followed vascular pathways in extinct dinosaurs 41 , that could explain why pneumatic internal structures in later, more derived dinosaurs resemble the ancestral, apneumatic structures documented in this study. A detailed study of the ontogenetic development of vertebral pneumaticity in a derived sauropod or non-avian theropod, using CT and bone histology, is needed to elucidate the relationship between pneumatic internal structures and the apneumatic structures that predate them (ontogenetically and phylogenetically). Eventually, the analysis of Norian bagualosaurian taxa will have the potential to provide evidence of the rise of unambiguous PSP, and the appearance of the invasive pneumatic system that we observe in later avemetatarsalians, including extant birds.

Conclusions
We analyzed three of the earliest dinosaurs and our data fill a gap in the knowledge of the evolution of the Respiratory System. Selected highlights are listed below: Microtomography of the three specimens indicates the absence of unambiguous postcranial pneumaticity. Therefore, we conclude that an air sacs system permeating the skeleton was not present in the earliest dinosaurs (late Carnian). The evidence of the absence of PSP in the earliest saurischian dinosaurs corroborates the hypothesis that the invasive air sac systems found in theropods, sauropods and pterosaurs were not homologous. This is solid evidence that unambiguous PSP evolved at least three times independently in those clades. Nonetheless, this still does not exclude the other hypothesis indicating that the homology of the underlying pulmonary tissue could be an ancestral ornithodiran condition. The chaotic organization of the larger internal trabecular vacancies and chambers, originally filled with blood and fat tissues, could have favored the evolution of the true pneumatic structures in the latest Triassic. Pampadromaeus was slightly younger than Buriolestes and already presented a pseudo-polycamerate structure, supporting the increasing complexity of these tissues that would later favor the origin of true unambiguous PSP.
Finally, investigations regarding Norian taxa are crucial in order to provide evidence on the rise of unambiguous PSP, and the appearance of the air sacs system as we know it today from the surviving avemetatarsalians (see Table S1 in supplementary materials).

Data availability
All fossils are housed in a public research institution and can be accessed with a request to the curator of the collection. Microtomography data are available upon request by email to the corresponding author.
Received: 3 October 2022; Accepted: 24 November 2022 Figure 8. A simplified cladogram of Avemetatarsalia illustrates the branches in which the unambiguous presence of an air sacs system has been evidenced (bright/green 'check' sign). The absence of an air sacs system in the oldest dinosaurs presented in this study eliminates the hypothesis of the homology of this trait between pterosaurs and saurischians. Our results also corroborate that this trait appeared independently in three clades: Pterosauria, Theropoda, and Sauropoda. "Early Sauropodomorpha" is a paraphyletic branch. Cladogram based on Novas et al. 54